The process of activation of cortical reaction is being examined in mouse, hamster and rabbit eggs in an attempt to understand the nature of activation triggered by the fertilizing sperm. Several membrane active agents, mostly inophors, are being screened for their ability to trigger cortical reaction and, a rapid assay for the cortical reaction is being developed. Preliminary results indicate the presence of an oveperoxidase on the surface of activated eggs. The peroxidase is believed to originate in the cortical granules and is apparently involved in the zona hardening process.